namespace: ignore prefix chars when comparing paths

Other callers of path_strv_contains() or PATH_IN_SET() don't seem to handle
paths prefixed with -+.

src/basic/path-util.h

@@ -173,3 +173,4 @@ static inline const char *empty_to_root(const char *path) {
 }
 
 bool path_strv_contains(char **l, const char *path);
+bool prefixed_path_strv_contains(char **l, const char *path);